Abnormalities of fetal growth are associated with decreased longevity of the offspring and the parents. This shorter life span is a result of increased mortality in the perinatal period, infancy, and adulthood. Low offspring birth weight is also associated with increased parental cardiovascular mortality. Large birth weight is associated with increased cancer-related mortality in the mother. Strong association of birth weight with mortality and morbidity results in clinical importance and scientific interest in fetal growth abnormalities. However, traditional methods of fetal growth assessment (population, ultrasound, and customized norms) have considerable limitations. They do not adjust for the effects of physiological determinants of fetal growth and pathological conditions. Adjustment for a very limited set of physiologic characteristics improved identification of perinatal mortality and other adverse pregnancy outcomes compared with population norms. However, those customized norms do not allow adjusting for African American and Hispanic race/ethnicity as well as a large number of proposed physiologic determinants of birth weight. Customized norms due to late ultrasound dating do not allow for identification of early growth abnormalities occurring in the first and second trimester. The norms were developed in a population including several complications associated with growth abnormalities. Those limitations markedly attenuate their accuracy and applicability. We have recently completed an observational study of 38,033 singleton pregnancies with first trimester ultrasound dating and comprehensive evaluation of pregnancy outcomes and exposures;this study provides us with an opportunity to overcome several of those limitations and develop individualized norms of optimal fetal growth and fetal growth potential norms. We hypothesize that individual optimal fetal growth norms are a better predictor of perinatal mortality and adverse pregnancy outcomes than population, ultrasound, and customized norms. Specific Aim 1: Identify physiologic determinants of birth weight, the characteristics associated with birth weight in a population of normal pregnancies. Based on those physiologic determinants, develop a predictive model of individual optimal birth weight. Specific Aim 2: Develop individualized norms of optimal fetal growth-fetal growth potential by combining individual optimal birth weight (Aim 1) and an equation of intrauterine fetal growth. Specific Aim 3: Validate fetal growth potential norms (Aim 2) by comparison with population, ultrasound, and customized norms in normal population, complicated population, and pregnancies with perinatal mortality. Clinically, accurate diagnosis of growth abnormalities may result in improved pregnancy outcomes, since effective interventions improving perinatal mortality in high-risk pregnancies exist. In clinical research, accurate identification of growth abnormalities will improve accuracy of results by decreasing false- positive and false-negative findings. Identifying physiologic determinants of fetal growth will also provide insight into mechanisms and abnormalities of human fetal growth.